PR ld/20828: Relax symbol ordering in tests

Complement commit 81ff47b3 ("PR ld/20828: Fix linker script symbols
wrongly forced local with section GC") and make tests check for the
presence of global `_fdata' and `_edata' symbols separately, removing
any dependency on symbol table ordering for tests to succeed and
removing:

FAIL: PR ld/20828 dynamic symbols with section GC (auxiliary shared library)
FAIL: PR ld/20828 dynamic symbols with section GC (plain)

failures with the `x86_64-solaris2' target, which has additional
intervening entries:

Symbol table '.dynsym' contains 6 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT    1 _fdata
     2: 0000000000000000     0 OBJECT  GLOBAL DEFAULT    1 _DYNAMIC
     3: 0000000000000000     0 OBJECT  GLOBAL DEFAULT  ABS _PROCEDURE_LINKAGE_TABLE_
     4: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT    1 _edata
     5: 00000000000001b8     0 OBJECT  GLOBAL DEFAULT    4 _GLOBAL_OFFSET_TABLE_

Rename dump pattern files accordingly for consistency.

	ld/
	PR ld/20828
	* testsuite/ld-elf/pr20828-1.sd: Remove test.
	* testsuite/ld-elf/pr20828-a.sd: New test.
	* testsuite/ld-elf/pr20828-2a.sd: Rename test to...
	* testsuite/ld-elf/pr20828-b.sd: ... this.
	* testsuite/ld-elf/pr20828-2b.sd: Rename test to...
	* testsuite/ld-elf/pr20828-c.sd: ... this.
	* testsuite/ld-elf/shared.exp: Adjust accordingly.

(cherry picked from commit adcbdc63)