core/alignment.h: Fix 'MBEDTLS_GCC_VERSION' is not defined error

[AmyChan-Intel]

Only check 'MBEDTLS_GCC_VERSION' when 'MBEDTLS_COMPILER_IS_GCC' is defined

Description

Only check 'MBEDTLS_GCC_VERSION' when 'MBEDTLS_COMPILER_IS_GCC' is defined

PR checklist

Please remove the segment/s on either side of the | symbol as appropriate, and add any relevant link/s to the end of the line.
If the provided content is part of the present PR remove the # symbol.

• changelog provided | not required because:
• framework PR provided Mbed-TLS/mbedtls-framework# | not required
• TF-PSA-Crypto development PR provided # | not required because:
• TF-PSA-Crypto 1.1 PR provided # | not required because:
• mbedtls development PR provided Mbed-TLS/mbedtls# | not required because:
• mbedtls 4.1 PR provided Mbed-TLS/mbedtls# | not required because:
• mbedtls 3.6 PR provided Mbed-TLS/mbedtls# | not required because:
• tests provided | not required because:

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[yanesca]

LGTM