x/build/cmd/release: linux-armv6l release tests aren't passing [1.15 backport]

[gopherbot]

@cagedmantis requested issue #43202 to be considered for backport to the next 1.15 minor release.

@gopherbot please consider this for backport to 1.15

[gopherbot]

Change https://golang.org/cl/326711 mentions this issue: [release-branch.go1.15] cmd/link: pass arch-specific flags to external linker when testing supported flag

[heschi]

Approved.

[gopherbot]

Closed by merging b4a2af2 to release-branch.go1.15.