powerpc/boot: Fix incorrect version calculation issue in ld_version

The ld_version() function computes the wrong version value for certain
ld versions such as the following:

  $ ld --version
  GNU ld (GNU Binutils; SUSE Linux Enterprise 15)
  2.37.20211103-150100.7.37

For input 2.37.20211103, the value computed is 202348030000 which is
higher than the value for a later version like 2.39.0, which is
23900000.

This issue was highlighted because with the above ld version, the
powerpc kernel build started failing with ld error: "unrecognized option
--no-warn-rwx-segments". This was caused due to the recent commit
579aee9f ("powerpc: suppress some linker warnings in recent linker
versions") which added the --no-warn-rwx-segments linker flag if the ld
version is greater than 2.39.

Due to the bug in ld_version(), ld version 2.37.20111103 is wrongly
calculated to be greater than 2.39 and the unsupported flag is added.

To fix it, if version is of the form x.y.z and length(z) == 8, then most
probably it is a date [yyyymmdd] commonly used for release snapshots and
not an actual new version. Hence, ignore the date part replacing it with
0.

Fixes: 579aee9f

 ("powerpc: suppress some linker warnings in recent linker versions")
Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
[mpe: Tweak change log wording/formatting, add Fixes tag]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20230104202437.90039-1-ojaswin@linux.ibm.com