cmd/link: pass the external linker's own arguments to flag probes

The external linker command may carry arguments of its own, as in
CC="ccache gcc" or the wrapper script used by TestScript/cgo_path_space_quote.
linkerFlagSupported ran only extld()[0], dropping those arguments, so a
wrapper that needs them failed and every probe reported "flag not
supported".

The consequential one is -no-pie: without it, a C toolchain that defaults
to PIE links non-PIC Go text into a PIE. On amd64 that only produces
DT_TEXTREL, but on ppc64le the absolute ADDR16_HA relocations become
dynamic relocations that ld.so cannot fit, and the program dies at startup
with "R_PPC64_ADDR16_HA reloc at 0x... for symbol `' out of range".

Pass the linker as an argv slice and forward linker[1:] to the probe.
Do the same for peHasLoadConfigDirectorySupport, and stop
passLongArgsInResponseFile from moving the linker's own arguments into the
response file, which the program named by argv[0] need not know how to
expand.

cmd/go already handles this correctly in gccSupportsFlag.

Should fix cmd/go TestScript/cgo_path_space_quote on ppc64,
where "$(which cc)" and "$(which cc) gcc" are different.

Fixes #81164.

Change-Id: Idad6ff5a31689b28f617e2985898bd732318cb02
Reviewed-on: https://go-review.googlesource.com/c/go/+/823024
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Matloob <matloob@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>