MIPS/BFD: Make section GC work with `ict_irix5' targets

MIPS/BFD: Make section GC work with `ict_irix5' targets

Prevent runtime procedure table symbols, produced with `ict_irix5' MIPS
targets, from being swept in section GC, fixing linker errors like:

./ld-new: tmpdir/dump: protected symbol `_procedure_table_size' isn't defined
./ld-new: final link failed: bad value

triggered whenever section GC is enabled with those targets and
consequently removing the following test suite failures:

FAIL: Build pr22649-2a.so
FAIL: Build pr22649-2c.so
FAIL: PR ld/20828 dynamic symbols with section GC (auxiliary shared library)
FAIL: PR ld/20828 dynamic symbols with section GC (plain)
FAIL: PR ld/20828 dynamic symbols with section GC (version script)
FAIL: PR ld/20828 dynamic symbols with section GC (versioned shared library)
FAIL: PR ld/20828 dynamic symbols with section GC (versioned)
FAIL: --gc-sections with .text._init
FAIL: pr20022

observed with `mips-elf', `tx39-elf', `mipsisa32-elf', `mipsisa64-elf',
`mipsel-elf', `mipsisa32el-elf', `mipsisa64el-elf', `mips64vr-elf',
`mips64vrel-elf', `mips64vr4300-elf', `mips64vr4300el-elf',
`mips-sgi-irix5' and `mips-rtems' targets, among others.  This fix makes
section GC usable with the affected targets.

	bfd/
	* elfxx-mips.c (_bfd_mips_elf_create_dynamic_sections): Set
	`mark' for symbols created from `mips_elf_dynsym_rtproc_names'
	list.