[flang] Add fastmath flags to localBuilder in IntrinsicCall

[flang] Add fastmath flags to localBuilder in IntrinsicCall

Currently the local builder used in IntrinsicCall doesn't have the
fastmath flags passed to it. This results in the fastmath attribute
not being added to certain runtime calls. This patch simply forwards
the fastmath flags from the parent builder.

Differential Revision: https://reviews.llvm.org/D154611