LD/testsuite: Pass $AFLAGS_PIC to GAS for PIC assembly

Add $AFLAGS_PIC flags for PIC assembly to a number of tests missing them
and remove `tic6x-*-*' XFAIL annotations from them, previously added to
paper over:

.../ld-new: warning: generating a shared library containing non-PID code

error messages produced due to `-mpic -mpid=near' GAS options having not
been used.  Such errors now do not happen anymore, removing:

XFAIL: Build shared library for pr14170
XFAIL: PR ld/21703 shared
XFAIL: Build shared library for broken linker script test
XFAIL: Build pr17068.so
XFAIL: -Bsymbolic-functions
XFAIL: Build pr20995.so
XFAIL: Build pr22374 shared library

with `tic6x-elf' and `tic6x-uclinux' targets.  These tests now pass all
except for:

FAIL: PR ld/21703 shared

which is now due to a different reason, as follows:

extra regexps in .../ld/testsuite/ld-elf/pr21703-shared.sd starting with "^Symbol table '\.dynsym' contains [0-9]+ entries:$"
EOF from dump.out
FAIL: PR ld/21703 shared

The addition of $AFLAGS_PIC requires the affected test cases to use the
`list' command rather than `{}' characters to create a list, to avoid
the quoting property `{}' also have in TCL.  Consequently the change is
slightly more extensive than it could otherwise be.

	ld/
	* testsuite/ld-elf/shared.exp: Add $AFLAGS_PIC throughout to PIC
	assembly builds where missing and remove `tic6x-*-*' XFAIL
	markings accordingly.