PR ld/20828: Remove leading `_' from symbols used in tests

Complement commit 81ff47b3 ("PR ld/20828: Fix linker script symbols
wrongly forced local with section GC") and remove the leading underscore
from `_fdata' and `_edata' symbols used in tests, fixing a:

FAIL: PR ld/20828 dynamic symbols with section GC (version script)

failure with targets such as: `bfin-elf', `bfin-uclinux', `metag-elf',
`metag-linux' `mn10300-elf', `sh-elf', `sh64-elf', and possibly other
ones, that have `_' set (with `elf_symbol_leading_char') as the leading
character for symbols.  As from commit 93252b1c ("bfd/ld: handle ABI
prefixes in version scripts") these targets strip the leading underscore
before applying version script rules, because the (default) syntax for
symbol names is that of the C language rather than their low-level
symbol table encoding.

	ld/
	PR ld/20828
	* testsuite/ld-elf/pr20828.ld: Rename `_fdata' and `_edata' to
	`fdata' and `edata' respectively.
	* testsuite/ld-elf/pr20828.ver: Adjust accordingly.
	* testsuite/ld-elf/pr20828-a.sd: Likewise.
	* testsuite/ld-elf/pr20828-b.sd: Likewise.
	* testsuite/ld-elf/pr20828-c.sd: Likewise.

(cherry picked from commit fda46c41)