postgresql: prevent addition of ARM target flag

configure.in checks for "ARMv8 CRC32C intrinsics" and goes as far as
adding "-march=armv8-a+crc" to the target flags if the compiler allows
it. This can clash with the OpenWrt target flags in
CONFIG_TARGET_OPTIMIZATION. If for example the latter is set to
"-mcpu=cortex-a9" the following warning is issued:

  cc1: warning: switch '-mcpu=cortex-a9' conflicts with '-march=armv5t' switch

This commit prevents configure.in from adding the mentioned flag. The
addition is unwanted when cross-compiling.

An issue was raised for this recently, see [1].

[1] https://github.com/openwrt/packages/issues/16034



Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>