arm64: Support running gen_vdso_offsets.sh with BSD userland.

arm64: Support running gen_vdso_offsets.sh with BSD userland.

BSD sed ignores whitespace character escape sequences such as '\t' in
the replacement string, causing this script to produce the following
incorrect output:

  #define vdso_offset_sigtrampt0x089c

Changing the hard tab to ' ' causes both BSD and GNU dialects of sed
to produce equivalent output.

Signed-off-by: John Millikin <john@john-millikin.com>
Link: https://lore.kernel.org/r/15147ffb-7e67-b607-266d-f56599ecafd1@john-millikin.com
Signed-off-by: Will Deacon <will@kernel.org>