ld: Force SHELL=/bin/bash in ld for Solaris [PR32580]

As described in PR ld/32580, when using SHELL=/bin/sh or /bin/ksh on
Solaris, the generated linker scripts get corrupted.  So far, the only
workaround is to enforce /bin/bash instead.

This is a major nuisance for developers and users alike, so this patch
automates this by overriding SHELL in ld/configure.ac.

Tested on amd64-pc-solaris2.11 in three configurations:

* CONFIG_SHELL unset

* CONFIG_SHELL=/bin/ksh

* CONFIG_SHELL='/bin/bash --norc'

In the first two cases, SHELL was set to /bin/bash as desired, while in
the third it was left unchanged.

2025-07-24  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>

	ld:
	PR ld/32580
	* configure.ac <*-*-solaris2*>: Enforce SHELL=/bin/bash.
	* configure: Regenerate.