aclocal.m4: narrow down 'riscv*' to 'riscv-*' and 'riscv32*'
aclocal.m4: narrow down 'riscv*' to 'riscv-*' and 'riscv32*'

Two minor changes:
- 32-bit CPU macro is called riscv32_HOST_ARCH (was riscv_HOST_ARCH)
- shrink understood tuples from riscv* to riscv-* and riscv32*-* as
  a tiny safeguard against riscv128*- in future.

Suggested-by: James Clarke <d29cd7184594bd12d9e96e534403edaa41d35c9d@jrtc27.com>
Signed-off-by: Sergei Trofimovich <110fdadc76b6bce16977f568bdb4723eaed6fb3f@gentoo.org>

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc