gas: accept leading zeros on dollar local labels in z80 sdcc compat mode

gas: accept leading zeros on dollar local labels in z80 sdcc compat mode

SDCC assembly output uses 5-digit numeric dollar sign labels, padded
with zeros. Commit 226749d made these invalid, and broke the Z80 SDCC
compatibility mode in GAS.

https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=226749d5a6ff0d5c607d6428d6c81e1e7e7a994b

This restores SDCC compatibility by replacing the leading zeros with
spaces when inside dollar local labels and when SDCC compatibility is
enabled. It also restores the SDCC test case to represent actual
syntax emitted by SDCC, and adds a note explaining the purpose of
the test.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33030