make sure struct real_value's bitfields are packed

[Alexandre Oliva]

REAL_VALUE_TYPE_SIZE is computed assuming the bitfields in struct
real_value are packed into a single 32-bit word, but some ABIs specify
that bitfields with different underlying types don't share the same
underlying word.

This patch arranges for all bitfields in struct real_value to
explicitly have type unsigned int.  exp was renamed to uexp and
accessor macros were introduced to turn it back into a signed value.
No performance differences are expected, since the macros effectively
do what the compiler would have done to sign-extend the values.

Bootstrapped on i686-pc-linux-gnu.  Ok to install?

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-- 
Alexandre Oliva             http://www.ic.unicamp.br/~oliva/
Red Hat Compiler Engineer   aoliva@{redhat.com, gcc.gnu.org}
Free Software Evangelist  oliva@{lsd.ic.unicamp.br, gnu.org}