[release-branch.go1.20] cmd/go: enforce flags with non-optional arguments

Enforce that linker flags which expect arguments get them, otherwise it
may be possible to smuggle unexpected flags through as the linker can
consume what looks like a flag as an argument to a preceding flag (i.e.
"-Wl,-O -Wl,-R,-bad-flag" is interpreted as "-O=-R -bad-flag"). Also be
somewhat more restrictive in the general format of some flags.

Thanks to Juho Nurminen of Mattermost for reporting this issue.

Updates #60305
Fixes #60512
Fixes CVE-2023-29404

Change-Id: I5989f68d21a8851d8edd47f08550850524ee9180
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/1876275
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
(cherry picked from commit 896779503cf754cbdac24b61d4cc953b50fe2dde)
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/1902226
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Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/1904346
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/501221
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