[mips] Remove absolute size assertion for end directive

The .end <symbol> directive for MIPS marks the end of a symbol and sets the
symbol's size. Previously, the corresponding emitDirective handler asserted
that a function's size could be evaluated to an absolute value at that point
in time.

This cannot be done with when directives like .align have been encountered,
instead set the function's size to the corresponding symbolic expression and
let ELFObjectWriter resolve the expression to an absolute value. This avoids
a redundant call to evaluateAsAbsolute.

llvm-svn: 294012