Speed up direct linking with DLLs on Windows (1/2).

This patch deals with the auto-import feature.  There are 2 versions
of this feature: the original one, which was piggybacked on the OS
loader with an optional help from the runtime (--enable-auto-import
--enable-runtime-pseudo-reloc-v1) and is still the one mostly
documented in the sources and manual; the enhanced one by Kai Tietz,
which is entirely piggybacked on the runtime (--enable-auto-import
--enable-runtime-pseudo-reloc-v2) and is the default for Mingw and
Cygwin nowadays.

The implementation is inefficient because of pe[p]_find_data_imports:
for every undefined symbol, the function walks the entire set of
relocations for all the input files and does a direct name comparison
for each of them.

This is easily fixable by using a hash-based map for v1 and a simple
hash table for v2.  This patch leaves v1 alone and only changes v2.
It also factors out pe[p]_find_data_imports into a common function,
removes old cruft left and right, and attempts to better separate
the implementations of v1 and v2 in the code.

ld/
	* emultempl/pe.em (U_SIZE): Delete.
	(pe_data_import_dll): Likewise.
	(make_import_fixup): Return void, take 4th parameter and pass it down
	in call to pe_create_import_fixup.
	(pe_find_data_imports): Move to...
	(gld_${EMULATION_NAME}_after_open): Run the stdcall fixup pass after
	the auto-import pass and add a guard before running the latter.
	* emultempl/pep.em (U_SIZE): Delete.
	(pep_data_import_dll): Likewise.
	(make_import_fixup): Return void, take 4th parameter and pass it down
	in call to pe_create_import_fixup.
	(pep_find_data_imports): Move to...
	(gld_${EMULATION_NAME}_after_open): Run the stdcall fixup pass after
	the auto-import pass and add a guard before running the latter.
	* pe-dll.c (runtime_pseudp_reloc_v2_init): Change type to bfd_boolean.
	(pe_walk_relocs_of_symbol): Rename into...
	(pe_walk_relocs): ...this.  Add 2 more parameters,4th parameter to the
	callback prototype and pass 4th parameter in calls to the callback.
	If the import hash table is present, invoke the callback on the reloc
	if the symbol name is in the table.
	(pe_find_data_imports): ...here.  Take 2 parameters.  Build an import
	hash table for the pseudo-relocation support version 2.  When it is
	built, walk the relocations only once at the end; when it is not, do
	not build a fixup when the symbol isn't part of an import table.
	Issue the associated warning only after a first fixup is built.
	(tmp_seq2): Delete.
	(make_singleton_name_imp): Likewise.
	(make_import_fixup_mark): Return const char * and a stable string.
	(make_import_fixup_entry): Do not deal with the pseudo-relocation
	support version 2.
	(make_runtime_pseudo_reloc): Factor out code and fix formatting.
	(pe_create_import_fixup): Add 5th parameter.  Clearly separate the
	pseudo-relocation support version 2 from the rest.  Fix formatting.
	* pe-dll.h (pe_walk_relocs_of_symbol): Delete.
	(pe_find_data_imports): Declare.
	(pe_create_import_fixup): Add 5th parameter.
	* pep-dll.c (pe_data_import_dll): Delete.
	(pe_find_data_imports): Define.
	(pe_walk_relocs_of_symbol): Delete.
	* pep-dll.h (pep_walk_relocs_of_symbol): Delete.
	(pep_find_data_imports): Declare.
	(pep_create_import_fixup): Add 5th parameter.
	* ld.texinfo (--enable-auto-import): Adjust to new implementation.