package/libxslt: fix bfin compile

package/libxslt: fix bfin compile

Fixes:
http://autobuild.buildroot.org/results/df4/df4bf247c92847024dff88ce58a5f47343f06258/
http://autobuild.buildroot.org/results/b33/b33f34dc23f7429928d7d0b1478e71d8e7fffed2/
http://autobuild.buildroot.org/results/cad/cad02570ec039511b9495e82d98439e9f0724b37/

And many more.

A GCC bug is triggered when the compiler is run with an optimization level
'-Os/-O2/-O3' for the Blackfin architecture:

```
error: unable to find a register to spill in class 'CCREGS'
```

Workaround this bug by forcing an optimization level of '-O1' when building
libxslt for the Blackfin architecture.

See GCC bug report:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77311

[Peter: add reference to autobuilders failures]
Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>