ld: fix alignment issue for ARM thumb long branch stub using PureCode section

ld: fix alignment issue for ARM thumb long branch stub using PureCode section

When pure-code option is activated. The linker creates for M-profile architecures
a 2-bytes branch instruction. This causes the section alignment to be set to 2-byte
alignment instead of 4-byte alignment. This is a problem for long branch stub
without pure-code section as it contains a 32-bit address as data, which is expected
to be 4-byte aligned. Hence creating a long branch stub for PureCode section followed
by a long branch stub will result in a misalignment for the 32-bit address.

An easy fix is to add a nop instruction after the branch to keep the section alignment
to 4 bytes.

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>
Co-authored-by: Guillaume VACHERIAS <guillaume.vacherias@st.com>