scripts/mkcompile_h: move LC_ALL=C to '$LD -v'

Minimize the scope of LC_ALL=C like before commit 87c94bfb ("kbuild:
override build timestamp & version").

Give LC_ALL=C to '$LD -v' to get the consistent version output, as commit
bcbcf50f

 ("kbuild: fix ld-version.sh to not be affected by locale")
mentioned the LD version is affected by locale.

While I was here, I merged two sed invocations.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>