Problem with .req directive in arm-elf-as (2.15 branch)

[Peter S. Housel]

The current binutils-2_15-branch has a problem with the ARM ".req"
pseudo-op.  The following code, which used to assemble correctly in 2.13,
now fails:

MMUPurgeTLBReg      .req c6
MMUCP               .req p15
MCR     MMUCP, 0, a1, MMUPurgeTLBReg, c0, 0

Apparently this results because the alias names are already downcased by the
time md_assemble() (and therefore create_register_alias()) in tc-arm.c get
to see them.

-Peter-